This invention relates to the application and curing of liquid coating materials, so as to produce on workplaces surfaces of thermoset or thermoplastic synthetic resinous materials.
There are many circumstances under which it is desirable to apply automatically a polymerizable liquid coating material to a workpiece, and to effect immediately thereafter curing of the liquid to a solid state. One field in which such a technique is beneficially employed involves the production of conformal coatings upon various workplaces, perhaps most notably printed circuit boards and printed wiring assemblies.
To be entirely satisfactory from a commercial standpoint, any system that is offered for producing such coatings must be fast, reliable, economical, and environmentally sound. It must be sufficiently flexible to be readily accommodated in existing production lines, and it must be capable of producing, with sufficient precision to satisfy industry standards, uniform and void-free coatings.
Conformal coatings have been produced heretofore by use of so-called "selective curtain coaters." Such apparatus employs a coating head that shuttles at high speed from place-to-place relative to the workpiece, and that is selectively activated so as to preferentially discharge coating material upon specific areas of the surface. Because of the precision afforded, these coaters make unnecessary the masking of workpieces that might otherwise be required to maintain selected portions of their surfaces coating-free. As a consequence of this precision, however, not only do such coaters tend to be inordinately complex and expensive for many applications, but moreover their proper functioning depends upon the secure positioning of the workpiece.
Aerosol spray techniques are also used in the production of conformal coatings. In operating such systems, however, a high proportion of the discharged coating material is lost to overspray, and there is also a strong tendency for air entrainment; the latter necessitates the allowance of a dwell-time for release of entrapped air, prior to curing of the deposit. In a paper entitled "The UV Curing Process: Applications For Conformal Coating and Adhesives In Electronic Assembly," Hnojewyj et al describe the use of pneumatically atomized spray coating machines for applying and curing UV-curable and heat-curable conformal coatings.
It is known as well in the art to use ultrasonic devices for applying liquids, as evidenced by the following United States patent properties: Sedlacsik, Jr. U.S. Pat. No. 2,881,092; Shizuo Nishiyama et al U.S. Pat. No. 3,114,654; Sedlacsik, Jr. U.S. Pat. No. 3,147,146; Toshio Onishi U.S. Pat. No. 3,198,174; Furgalus et al U.S. Pat. No. 3,966,120; LaBianca U.S. Pat. No. 4,806,455 and Sadler et al SIR No. H153. U.S. Pat. Nos. 4,460,619 and 4,495,885, to Derks et al, disclose an apparatus and method for transporting and depositing viscous materials, in accordance with which the components of a two-part adhesive may be supplied to the opposite sides of an ultrasonically vibrating sonotrode; the ingredients are transported along the sonotrode to the free end of a pin, where they are mixed and thereafter deposited in dropwise fashion.
Drummond patent U.S. Pat. No. 2,450,503 teaches that two substances (i.e., a catalyst and a polymerizable hydrocarbon) may be sprayed simultaneously to intermix just prior to contact with the surfaces being treated. In accordance with Harrison et al U.S. Pat. No. 3,676,197, sprays of (for example) a polyester resin and a peroxide catalyst are caused to converge prior to impinging upon a surface; the patents to Self, U.S. Pat. No. 4,013,614, and to Takegawa et al, U.S. Pat. No. 4,479,840, are similar. Spraying of a catalyst upon a monomer coating is disclosed by Quinlidan, in U.S. Pat. No. 2,684,305.
Despite the activity in the art indicated by the foregoing, the need exists for a fast, reliable, economical, relatively incomplex and environmentally attractive system and method by which a liquid coating material can be applied to one or a multiplicity of separate workpieces, and subsequently cured, so as to produce a high-quality, uniform, and void-free coating thereon, which system and method are readily adapted to existing operations and are sufficiently precise to satisfy applicable coating specifications.